1. Field of the Invention
The preferred embodiments are directed to a high speed scanning probe microscope (SPM), and more particularly, a Z-scanner assembly for an SPM which is compact and lightweight, as well as readily removable from a head of the SPM to facilitate ease of use, while maintaining SPM performance, including fast scanning.
2. Discussion of the Prior Art
A scanning probe microscope, such as an atomic force microscope (AFM) operates by providing relative scanning movement between a measuring probe and a sample while measuring one or more properties of the sample. A typical AFM system is shown schematically in FIG. 1. An AFM 10 employing a probe device 12 including a probe 14 having a cantilever 15. Scanner 24 generates relative motion between the probe 14 and sample 22 while the probe-sample interaction is measured. In this way images or other measurements of the sample can be obtained. Scanner 24 is typically comprised of one or more actuators that usually generate motion in three orthogonal directions (XYZ). Often, scanner 24 is a single integrated unit that includes one or more actuators to move either the sample or the probe in all three axes, for example, a piezoelectric tube actuator. Alternatively, the scanner may be an assembly of multiple separate actuators. Some AFMs separate the scanner into multiple components, for example an XY scanner that moves the sample and a separate Z-actuator that moves the probe.
In a common configuration, probe 14 is often coupled to an oscillating actuator or drive 16 that is used to drive probe 14 at or near a resonant frequency of cantilever 15. Alternative arrangements measure the deflection, torsion, or other motion of cantilever 15. Probe 14 is often a microfabricated cantilever with an integrated tip 17.
Commonly, an electronic signal is applied from an AC signal source 18 under control of an SPM controller 20 to cause actuator 16 coupled to the probe holder wedge (or alternatively scanner 24) to drive the probe 14 to oscillate. The probe-sample interaction is typically controlled via feedback by controller 20. Notably, the actuator 16 may be coupled to the scanner 24 and probe 14 but may be formed integrally with the cantilever 15 of probe 14 as part of a self-actuated cantilever/probe.
Often a selected probe 14 is oscillated and brought into contact with sample 22 as sample characteristics are monitored by detecting changes in one or more characteristics of the oscillation of probe 14, as described above. In this regard, a deflection detection apparatus 25 is typically employed to direct a beam towards the backside of probe 14, the beam then being reflected towards a detector 26, such as a four quadrant photodetector. Note that the sensing light source of apparatus 25 is typically a laser, often a visible or infrared laser diode. The sensing light beam can also be generated by other light sources, for example a He—Ne or other laser source, a superluminescent diode (SLD), an LED, an optical fiber, or any other light source that can be focused to a small spot. As the beam translates across detector 26, appropriate signals are transmitted to controller 20, which processes the signals to determine changes in the oscillation of probe 14. In general, controller 20 generates control signals to maintain a relative constant interaction between the tip and sample (or deflection of the lever 15), typically to maintain a setpoint characteristic of the oscillation of probe 14. For example, controller 20 is often used to maintain the oscillation amplitude at a setpoint value, AS, to insure a generally constant force between the tip and sample. Alternatively, a setpoint phase or frequency may be used.
A workstation 40 is also provided, in the controller 20 and/or in a separate controller or system of connected or stand-alone controllers, that receives the collected data from the controller and manipulates the data obtained during scanning to perform point selection, curve fitting, and distance determining operations. The workstation can store the resulting information in memory, use it for additional calculations, and/or display it on a suitable monitor, and/or transmit it to another computer or device by wire or wirelessly. The memory may comprise any computer readable data storage medium, examples including but not limited to a computer RAM, hard disk, network storage, a flash drive, or a CD ROM. Notably, scanner 24 often comprises a piezoelectric stack (often referred to herein as a “piezo stack”) or piezoelectric tube that is used to generate relative motion between the measuring probe and the sample surface. A piezo stack is a device that moves in one or more directions based on voltages applied to electrodes disposed on the stack. Piezo stacks are often used in combination with mechanical flexures that serve to guide, constrain, and/or amplify the motion of the piezo stacks. Additionally, flexures are used to increase the stiffness of actuator in one or more axis, as described in copending application Ser. No. 11/687,304, filed Mar. 16, 2007, entitled “Fast-Scanning SPM Scanner and Method of Operating Same.” Actuators may be coupled to the probe, the sample, or both. Most typically, an actuator assembly is provided in the form of an XY-actuator that drives the probe or sample in a horizontal, or XY-plane and a Z-actuator that moves the probe or sample in a vertical or Z-direction.
As the utility of SPM continues to develop, a need has arisen for imaging different types of samples at greater speeds to improve sample measurement throughput (e.g., more than 20 samples per hour) and/or measure nanoscale processes with higher time resolution than currently available. Although AFM imaging provides high spatial resolution (nanoscale), it has generally low temporal resolution. Typical high quality AFM images take several minutes to acquire, especially for scan sizes above a few microns.
Several factors can limit imaging speed, including the cantilever response time, the usable scanner bandwidth in X, Y and Z directions, the power and bandwidth of the high voltage amplifier that drives the scanner, the speed of the cantilever force sensing, as well as the demodulation system and the tracking force feedback system.
As with most measuring devices, AFMs often require a trade off between resolution and acquisition speed. That is, some currently available AFMs can scan a simple surface with sub-angstrom resolution. These scanners are capable of scanning only relatively small sample areas, and even then, at only relatively low scan rates. Traditional commercial AFMs usually require a total scan time typically taking several minutes to cover an area of several microns at high resolution (e.g. 512×512 pixels) and low tracking force. The practical limit of AFM scan speed is a result of the maximum speed at which the AFM can be scanned while maintaining a tracking force that is low enough not to damage or cause minimal damage to the tip and/or sample. Professor Toshio Ando at Kanazawa University in Japan has made tremendous progress with high-speed AFM using an AFM that scans mm-sized samples over small distances, typically less than 2 um. Professor Ando has achieved video scan rates with high resolution for this combination of small samples and small scan sizes.
Other systems, typically called “tip scanners,” are known or have been proposed and/or implemented in which the probe is mounted on the scanner. One such system is incorporated in a line of instruments marketed by Veeco Instruments under the name Dimension®. That system employs a relatively massive tube scanner for the Z-actuator and has relatively low bandwidth. Another system is disclosed in U.S. Pat. No. 7,249,494 to Hwang. In the system of the Hwang application, the probe is mounted on an actuator that, in turn, is mounted on an optical objective that focuses incoming laser light. The objective, in turn, is mounted on an x-y actuator. However, because the objective and other optics of the system are fixed relative to the probe, relatively large probes (having a width of at least of 20 μm, length of more than 40 μm) are required to assure positioning of the focused laser beam on the cantilever. The typical probes used also have a resonant frequency Fo of roughly 400 kHz and a quality factor Q of around 400. The resulting response bandwidth for these probes is of the order of Fo/Q≈1 kHz. Due in part to its low-bandwidth probe, the resulting system has a maximum scan rate of less than 30 Hz (or 30 scan lines per second), and more typical imaging speeds are around 1 Hz.
On the other hand, SPMs that can acquire data rapidly can also suffer unacceptable tradeoffs. One such system is marketed by Infinetisma under the name Video AFM™. The Video AFM operates at video rates but with significant compromises to signal-to-noise ratio and resulting image quality. The Infinitesima system also operates in contact mode with force feedback that is not fast enough to respond to variations in sample corrugation within a scan line. In this system, the sample or the probe is mounted on a tuning fork. The probe is driven into contact with the sample while the sample or the probe is scanned by vibrating the tuning fork at or near its resonant frequency. Because the tuning forks need to be quite small (typically on the order of a few mm in size) to achieve high resonant frequencies, they are very sensitive to being loaded by extra mass. As a result, only very small (on the order of a few mm in size) samples or cantilever substrates can be mounted to the tuning fork without degrading the performance.
It is known to combine an AFM with a conventional optical microscope to provide a view of the surface features of the sample. Notably, high performance microscope objectives have a short working distance and must be positioned close to the sample surface. High resolution optical imaging is therefore difficult to implement in combination with traditional AFM detectors because there is insufficient space between the bottom of the objective and the probe to accommodate the geometry for the incoming and outgoing detection beams. Because of the weight of the optical microscope, it is difficult to incorporate the optics of an optical microscope into the scanner of the AFM without unacceptably reducing the instrument's scan rate.
Some optical microscope-equipped SPMs have attempted to overcome this limitation by directing laser light through the microscope objective. One such system has been commercialized by Surface Imaging Systems under the name ULTRAOBJECTIVE™ and is disclosed in international publication number WO 01/23 939. In the ULTRAOBJECTIVE™ system, a near field AFM probe, a z actuator assembly for the probe and optical focusing system are provided in a single housing in order to provide an interchangeable objective that can be inserted in the objective turret of an optical microscope. Its objective is fixed relative to the probe, and it lacks any mechanism for dynamically focusing the laser beam onto the probe.
Another drawback of conventional optical microscope equipped AFMs is that the optical microscope is provided only to allow the user to inspect the sample. It plays no role in focusing the laser beam on the cantilever. Hence, even if the system were provided for focusing the light spot on the cantilever, no mechanism would be available to provide the user with optical feedback during a focusing process.
Solutions in this regard are available. However, two major drawbacks to maintaining a large first or fundamental resonant frequency for all moving components of an AFM include 1) the size and mass of the scanner and probe holder, and 2) the non-rigid coupling of these components to the AFM head, and each other.
As illustrated in FIGS. 2 and 3, a large probe holder 50 is often employed in conventional AFMs. In one type of AFM, a piezoelectric tube actuator is supported within an AFM head that supports coupling pins at its distal end to accommodate a probe holder such as probe holder 50. Probe holder includes a relatively massive body 52 with mounting apertures 62 formed therein to receive the mounting pins. A wedge 54 is provided on a surface of body 52 and configured to provide cantilever angle and support. An arm 58 is provided to hold the base of a probe device 56 supported by wedge 54. A screw 60 holds arm 58 to wedge 54. A user pushing on a back end of arm 58 operates to release the probe for installation, removal and replacement. Overall, this is a massive structure, which together with the scanner itself occupies a large volume (10s of cubic inches). This moving structure significantly limits the mechanical resonance of the AFM and thus AFM scanning speed.
Other AFM scanning solutions are available, but each has drawbacks as well. Even those that are capable of maintaining high fundamental resonance frequency often are unwieldy, and thus are difficult to use. In particular, picking up the relatively massive head and scanner, and turning it over and placing it so that probes may be mounted, removed and replaced is often a challenge. In this regard in particular, it is notable that the probe device is a consumable that often needs to be replaced on an hourly basis, and sometimes over much shorter time spans.
The field of scanning probe microscopy was thus in need of a scanner with improved rigidity and smaller mass and readily detachable to facilitate probe exchange. A smaller probe holder, having a higher first resonant frequency, was also desired. In the end, the scanner that facilitates easy of use and fast AFM operation would be ideal.